The present invention relates, in general, to an extrusion plant of a type having several drive units, and to a method of synchronizing the speeds of the drive units.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
Extrusion plants are typically used for extruding thermoplastic material. A thermoplastic starting material in the form of pellets or powder is hereby fed by a metering unit into a conveying screw which homogenizes and thermally plasticizes the material. Plasticized material is pressed through a shaping die, producing a strand which is drawn from a take-off unit through cooling units. Extrusion plants of this type are applicable for producing profiled bars, tubes, plates and the like. The extrusion plant which may be comprised of several extruder units has drive units that are normally centrally controlled and may involve plasticizing screw, metering unit, and take-off unit, for example.
In order to provide a reproducible product quality, any change in speed of the extrusion plant requires a synchronous adjustment of the speeds of all drive units. One synchronization approach involves a ratio-based translation of the speed change of a drive unit to the other drive units. This is problematic when the speed is adjusted frequently because inaccuracies of the numerical computation result in a divergence of the drive speeds. Another synchronization approach allows synchronization only when a minimum speed has been reached because otherwise the coupling factors between the individual drive units cannot be calculated accurately enough. Synchronization is thus ineffective, when the speed drops below the minimum speed, so that synchronization must be executed by hand.
It would therefore be desirable and advantageous to provide an improved method of synchronizing drive units and to provide an improved extrusion plant with synchronized drive unit, to obviate prior art shortcomings and to attain a continuous and lasting synchronization across the entire speed range.